1. Field of the Invention
The present invention relates to a semiconductor device having sense amplifiers.
2. Description of Related Art
One typical semiconductor device having sense amplifiers is a dynamic semiconductor memory device, for example. The semiconductor memory device has sense amplifiers connected to bit line pairs connected to memory cells. When data are read from a memory cell, the sense amplifier amplifies a small potential difference output from the memory cell to the bit line pair. As such semiconductor memory devices are integrated to a higher degree, their storage capacity increases, but they are required to reduce electric power consumption and to operate at higher speeds.
For reducing the electric power consumption of a semiconductor memory device, it is effective to lower the operating voltage of internal circuits thereof. For example, if an external power supply voltage of +1.8 V is applied to a semiconductor memory device, then a step down circuit in the semiconductor memory device generates an internal power supply voltage of +1.05 V, for example, from the external power supply voltage, and the internal power supply voltage is supplied as an array voltage VARY to a memory cell array. The memory cell array comprises a plurality of memory cells, a decoder accessing the memory cells, and sense amplifiers sensing memory cell information. The array voltage VARY is a charging voltage for the bit lines, i.e., an equalizing voltage. If the array voltage VARY is lowered to lower the charging potential for the bit lines, then according to the known ½ equalizing scheme, the potential of the power supply for the sense amplifier is lowered, and the sensing time is increased. The sensing time refers to a time required after the sense amplifier has started to amplify the small potential difference on the bit line pair until the potential between the bit lines of the bit line pair increases to a prescribed potential difference which is regarded as being of a substantially constant value. The prescribed potential difference is of a value which is 95% of the array voltage VARY, for example.
For reading data from a semiconductor memory device, it is the general practice to make a word line active to select a memory cell and transmit its information to bit lines, starting to operate a sense amplifier, and thereafter to select a Y-selection signal line with a Y-decoder at the time the output from the sense amplifier is established, decoding an output potential difference from the sense amplifier. If the sensing time is increased, then as the potential difference between the bit lines is read when the potential difference is not yet sufficiently large, the potential difference cannot properly be read. Consequently, it is necessary to increase the time required after the word line is selected until the Y-decoder selects the Y-selection signal line. This means a reduction in the rate at which to read data from the semiconductor memory device to an external circuit.
In order to reduce the sensing time and realizing high-speed operation of the semiconductor memory device, it has been customary to supply an overdrive voltage VOD higher than the array voltage VARY (VOD>VARY) to the sense amplifier to energize the sense amplifier with the overdrive voltage VOD in an initial stage of the operation of the sense amplifier. Such a process is referred to as an overdrive technique. If the array voltage VARY is +1.05 V, for example, then the overdrive voltage VOD is set to about +1.4 V.
If the semiconductor memory device is of the dynamic type, then in order to hold or maintain the data in the memory cells, it is necessary to periodically perform a refreshing mode to restore the same data in the memory cells by operating the sense amplifiers on the memory cells to periodically read the data. Unlike reading the data in the semiconductor memory device from an external circuit, however, the refreshing mode is carried out simultaneously on the many memory cells of the semiconductor memory device. Consequently, if the overdrive voltage is applied in the refreshing mode, the peak currents which flow while the sense amplifiers are in operation increase, resulting a reduction in the internal voltage and the generation of noise. To solve these problems, JP-2003-68073A discloses that a lower overdrive voltage is used in the refreshing mode than when the data in the semiconductor memory device are read from an external circuit. Specifically, when the semiconductor memory device is in a normal mode of operation to output the stored data to the external circuit, the external power supply voltage is used as the overdrive voltage, and when the semiconductor memory device is in the refreshing mode and does not output the stored data to the external circuit, a second overdrive voltage which is generated from the external power supply voltage by a step down circuit and lower than the external power supply voltage is applied to the sense amplifiers.
As described above, semiconductor devices, typically semiconductor memory devices, use an overdrive voltage in an initial stage of the operation of the sense amplifiers.
For reducing the electric power consumption of semiconductor devices, efforts are being made to lower the external power supply voltage that is applied to the semiconductor devices. For example, it has been attempted to use an external power supply voltage of +1.2 V for dynamic semiconductor memory devices. However, though an array voltage VARY of +1.05 V can be generated from the external power supply voltage of +1.2 V by a step down circuit, an overdrive voltage of about +1.4 V cannot be generated from the external power supply voltage of +1.2 V by a step down circuit. Therefore, a step up circuit such as a charge pump circuit mounted on the semiconductor chip is used to generate a voltage of about +1.8 V from the external power supply voltage, and then an overdrive voltage of about +1.4 V is generated from the voltage of about +1.8 V by a step down circuit.
When the overdrive voltage is generated from the external power supply voltage by the charge pump circuit, the pump efficiency of the charge pump circuit becomes about 2.5 times under the above voltage conditions, and the current consumed when the sense amplifiers are in operation increases 2.5 times. Though a step up circuit other than the charge pump circuit may be used, such a step up circuit still causes a large increase in the current consumed when the sense amplifiers are in operation.
As described above, if an external power supply voltage supplied to a semiconductor device with sense amplifiers is lowered, it is necessary for the semiconductor device to have a step up circuit for generating an overdrive circuit. However, the step up circuit causes a large increase in the electric power consumption of the semiconductor device.